It will be hot. You will wait in long lines. Somebody will spill a drink on you. There will not be enough cabs.

If it rains, don’t even think about bringing an umbrella to the convention hall. To security people, umbrellas and bazookas are the same thing.

In the hall, it will be very, very noisy. Somebody with a wedge of cheese on his head will block your view. There will not be enough washrooms. Somebody will spill another drink on you.

And more than once, you will think to yourself, “I could have stayed home and seen this better on TV.”

Which is true. But, in the end, those who actually go to the conventions will be glad they did, even though they may never be able to pinpoint exactly why.

Presidential nominating conventions are models of controlled hysteria. They are relentlessly slick and obsessively noncontroversial. They lack both surprise and necessity. They are vulgar and silly and very, very American.

About 15,000 journalists will watch about 5,500 Democratic delegates and about 2,300 Republican delegates cast symbolic votes for two nominees who have already been selected.

It is like covering the Oscars after the envelopes have already been opened.

But we will find something to write and talk about. We always do. While the drama has been reformed away — party primaries and caucuses now decide the nominees months before the conventions begin — the good times remain.

Political parties aren’t called parties for nothing.

But while the conventions are technically organized by the parties, they are actually controlled by the nominees, who rigidly vet and edit every speech before it is delivered.

Then-Illinois state Sen. Barack Obama gave a stirring and career-making keynote address at the Democratic National Convention in Boston in 2004. But John Kerry was the nominee, and Kerry’s speechwriters demanded that Obama cut a key line from his speech because it was too similar to a line Kerry wanted to use. According to David Bernstein of Chicago magazine, Obama was livid. “That f——- is trying to steal a line from my speech!” Obama fumed to his chief aide, David Axelrod.

Axelrod advised calm. “They had given him this great opportunity,” Axelrod said. “Who was he to quibble over one line?”

The Kerry people had also told Obama he would be allotted just eight minutes for his speech — which was a problem, since Obama’s first draft clocked in at 25 minutes.

Tastes and attention spans had changed over the years. Alben Barkley, a 70-year-old senator from Kentucky and a Democratic war horse, had delivered the keynote addresses at the 1932 and 1936 Democratic conventions and was tapped to do the same at the 1948 convention.

Writing from under his favorite apple tree in Paducah, Barkley came up with a 105-minute draft, which he cut down to 68 minutes as he delivered it. These days, no TV director in his right mind would stick with a 68-minute keynote address. But TV was very, very new in 1948 and speakers didn’t give much of a hoot about it. They were speaking to the people in the hall.

And the delegates in the convention hall in Philadelphia were held spellbound by Barkley’s address. “Behold!” he concluded. “Civilization knocks at the door! Shall we hear the voice and open the door? Or shall we … refuse to lead the children of men out of the bondage of fear and slavery into a free world and a free life?”

Today, such highfalutin verbiage would be met with giggles. But in 1948, the delegates went wild and clogged the aisles with a 28-minute demonstration, cheering madly for Barkley. Watching Barkley’s speech from the White House on a recently installed 12-inch DuMont television — one of the largest televisions in existence — President Harry Truman made a decision. Truman had wanted Supreme Court Justice William O. Douglas as his running mate, but it was clear the convention wanted Barkley. And Truman was not going to buck the will of the convention. (The Truman and Barkley ticket would go on to win that November.)

Slowly, but inexorably, conventions lost the power to express their “wills.” They no longer had wills. Delegates on the floor became mere backdrops, stand-ins, extras for the TV picture. And TV demanded shorter, snappier speeches.

When, in 1988, then-Arkansas Gov. Bill Clinton gave an excruciating 32-minute speech nominating Michael Dukakis, he was roundly condemned and had to go on Johnny Carson and play the saxophone to show that he was really a regular guy.

In 2004, the Obama people finally would hand the Kerry people a 17-minute speech — which would take Obama 21 minutes to deliver — and Obama was met with national acclaim. Four years later, Obama would be the nominee. And then president. And that keynote address had begun the process. Convention speeches could still make a difference. They could still change history.

But conventions had to be controlled. No nominee was going to be stampeded into picking a running mate he did not want. And no nominee wanted anything negative or even controversial at the convention.

By 1996, conventions had become so scripted that they were also sleep-inducing. In San Diego, the Republicans put on a convention so sanitized that the word “abortion” was banned from use on the podium. The delegates argued madly over a platform — delegates always do — but nominee Bob Dole didn’t even bother to read it. Platforms didn’t matter anymore.

Ted Koppel and his “Nightline” crew made headlines when they packed up in the middle of the convention and went home. “This convention is more of an infomercial than a news event,” Koppel said. “Nothing surprising has happened. Nothing surprising is anticipated.”

The only thing that surprised me at the time was that Koppel had come to San Diego expecting to be surprised. What universe did he live in? Surprise is what the media always want and what conventions always strive to eliminate.

Why do the media show up at all? Good question, and one that TV has been wrestling with for several cycles now. Prime-time commercial network coverage of conventions has been cut way back over the years, and some networks always threaten to drop live coverage entirely.

For a few days in late summer, however, the conventions are still the greatest show on Earth. And the delegates and the media show up in the hall, and the people watch and listen at home, because anything can happen.